Preheat oven to 350F. Line an 8- or 9-inch square baking pan with aluminum foil and lightly grease.
In a small, heatproof bowl, melt butter and chocolate together. Stir with a fork until very smooth. Set aside to cool for a few minutes.

In a large bowl, whisk together sugar, eggs, vanilla extract and red food coloring. Add in the chocolate mixture and stir until smooth. Batter should be red. If a brighter red is desired, add an additional 1/2 tsp food coloring. Add flour and salt into the bowl and stir until everything is just combined and no streaks of dry ingredients remain.